Frequency variation response circuit



July 8, 1941. K. RATH FREQUENCY VARIATION RESPONSE CIRCUIT Filed Feb. 14, 1940 2 Sheets-Sheet 1 INVENTOR KARL RAT H W K ATTORNEYJ Patented July 8, 1941 FREQUENCY VARIATION RESPONSE CIRCUIT Karl Bath, New York, N. Y., assignor to Radio Patents Corporation, a corporation of New York Application February 14, 1940, Serial No.'3l8,972

(Cl.2502'l') 11 Claims.

wave upon a tuned circuit which yields a varying amplitude in response to the frequency variations to be detected according to whether the frequency approaches or recedes from the resonance frequency 'of the circuit. The resulting amplitude modulated wave is then demodulated or detected in the usual manner by employing any one of the known demodulators or detecting devices or systems.

The shape of the resonance curve of the usual tuned circuit is such that the sides of the curve are not linear and particularly towards the lower portion the curvature introduces a substantial non-linearity resulting in considerable distortion in the resulting amplitude modulation. Moreover, the tuning adjustment of a resonant circuit is subject to constant changes .and fluctuations or drift caused by varying temperature or other influences. acteristics of a tuned circuit render this method of detecting frequency moduated signals substantially unsuited for most practical purposes in particular for use in present day wide band frequency modulation broadcast and other receivers.

Another known system for detecting frequency variations as used in the discriminator circuits in automatic frequency control systems for broadcast receivers and the like or as demodulating circuits in frequency modulation receivers substantially comprises a pair of inductively coupled resonant circuits constituting a band-pass filter network tuned to the carrier frequency whereby the primary and secondary potentials deviate in either direction from a normal 90 relative phase angle in proportion to the frequency departure of the impressed potential above and below, respectively, the tuning (carrier) frequency to which the network is resonant. By forming the sum and difference of these potentials, by connecting the center tap of the secondary circuit to the high potential side of the primary circuit and subsequent rectificationof the'sum and difference potentials,

These properties and chara frequency responsive potential is obtained by differentially combining the rectified potentials which may serve as a tune correcting potential in an AFC system, or as the demodulated signal in a frequency modulation receiver.

This arrangement also suffers from several disadvantages which seriously impair its efilciency. Thus, the center tap of the secondary winding of the tuned band-pass filter has to be adjusted accurately or correct balancing of the potentials'at resonant frequency is not obtained resulting in distortions and other defects. Other disadvantages are due to the mutual coupling of the circuits making it difficult to both adjust and maintain the tuning due to the reaction of either circuit upon the other circuit in addition to various other defects well known.

Accordingly, an object of the present invention is to improve generally the simplicity and efficiency of high frequency variation response networks and to provide a novel frequency discriminating or detecting circuit which will not only be reliable in operation but which may be readily manufactured and assembled in known signalling apparatus.

Another object is to provide a frequency variation response or detecting circuit for converting frequency modulated waves into corresponding amplitude modulated waves to be detected in the usual manner employing circuits for producing potentials of varying phase in response to the frequency departure from a normal or fixed frequency, said circuits being coupled substantially uni-laterally only to prevent mutual interaction and to allow accurate and permanent adjustment in operation.

A further object is to provide a detecting system for frequency modulated waves which shall be unresponsive to the carrier wave of the normal unmodulated frequency.

The above and further objects and novel aspects of the invention will become more apparent from the following detailed description taken with reference to the accompanying drawings forming part of this specification and wherein:

Figure 1 is a circuit diagram illustrating a frequency variation response system embodying .18 features of the invention,

Figure 2 is a modification of a frequency variation detector or response circuit constructed according to principles of the invention.

Figure 3 isa circuit diagram of a complete frequency modulation receiver with the improved detector circuit according to the invention embodied therein.

Like reference characters identify like parts in the different diagrams of the drawings.

With the aforementioned objects in view, the invention in one of its uses involves the employment of an electron discharge device comprising means for producing an electron space current and having at least one control grid electrode located in the path of the space current. Further means such as an additional control grid are provided for controlling the electron space current in accordance with a potential or signal of Varying frequency to be detected or demodulated such as a frequency modulated radio signal or any other alternating potential of varying frequency. According to the invention, the discharge device is further provided with means such as a positively biased screen grid electrode to produce a concentrated space charge or virtual cathode adjacent to the above control grid. A resonant circuit tuned to a predetermined frequency preferably the unmodulated or carrier frequency is connected to this control grid but otherwise substantially decoupled from the system in such a manner as to be excited by reactive (capacitative) coupling with said. space charge at varying phase depending upon the relative frequency departure between the signal frequency and the resonant frequency to which said resonant circuit is tuned. For the carrier frequency the potential on the control grid is then 90 out of phase with the variations of the space current at signal frequency due to the capacitative coupling of the resonant circuit with the virtual cathode, thereby providing no component either in phase or in anti-phase with the variations of the space current at signal frequency and accordingly have no effect upon the output current variations at signal frequency. If the signal frequency deviates in either direction from the frequency of the tuned circuit, then the grid potential will be de-phased from the space current fluctuations at signal frequency by an amount greater or lesser, respectively, than 90 depending on the sense of the frequency deviation in such a manner as to provide a component either in phase or in counterphase with the space current fluctuations at signal frequency, thus increasing and decreasing, respectively, the output current fluctuations.

-As a result, the output current variations at the signal frequency will be modulated in amplitude in accordance with the origina1 frequency modulation. This amplitude modulation is detected by the aid of any known demodulator or detecting device or system, for example a linear (diode) detector resulting both in increased efiiciency and fidelity of the frequency conversion compared with demodulating arrangements known in the prior art.

Referring to Figure 1, there is shown a frequency response or conversion system for use in AFC systems, frequency modulation receivers or the like, only those parts being shown which are necessary for an understanding of the invention. An input signal wave of varying frequency such as a frequency modulated radio signal is impressed by way of input terminals ab and a coupling transformer Ill having its primary and secondary windings shunted by condensers II and II, respectively, in a manner well known and having substantially flat frequency response over the frequency variation range (provision of a band spread resistance shunting the primary and/or secondary circuits) cathode path of a mixer or converter electron tube I2 being of the hexode type in the example shown and comprising a cathode I3, an input control grid I4, a positively biased screen grid I5, a second control or injector grid I6, and an anode or plate I1. A biasing network I8 comprising a, resistance by-passed by a condenser is inserted between the cathode I3 and ground 39 or any other zero potential point of the system to provide proper operating grid bias potential. A tuned circuit comprising an inductance coil I9 shunted by a condenser 20 is connected between the second control grid I6 and ground and excited in accordance with the signal potential impressed upon the input grid I4 substantially by electron coupling with the space charge or virtual cathode formed between the screen grid I5 and the grid I6 at varying phase in accordance with the frequency departure of the impressed potential from the resonant frequency to which the tuned circuit Iii-20 is resonant. There is thus obtained in the manner described hereinbefore in the output or plate circuit of the tube I2 a high frequency signal having its amplitude modulated according to the original frequency modulation which signal is impressed upon a subsequent combined amplifier and rectifier tube 25 by way of a coupling transformer 2| having its secondary tuned by means of a condenser 22 of design well known to those skilled in the art so as to have substantially aperiodic characteristics over the operating frequency variation range (provision of band spread resistance 22' shunting the secondary circuit shown in the drawings). The transformer secondary is connected across the grid-cathode path of the tube 25 by way of a grid coupling condenser 23 and a grid leak resistance 24. The tube 25 in the example shown is of the pentode type comprising a cathode 26, input grid 21, screen grid 28, suppressor grid 29 internally connected to the cathode 26 and plate 30. Thus, demodulation of the amplitude modulated signal wave impressed upon input grid 21 is accomplished by grid leak detection by the action of the leak resistor 24 and grid condenser 23. Any other method of detectionmay, however, be employed for the purpose of this invention as is understood, such as plate detection by omitting the grid condenser 23 and grid leak resistance 2G and suitably biasing the grid 21 such as by a biasing network in the cathode lead so as to operate at a point of the curved portion of the grid potential-anode current characteristic in a manner well known. The demodulated signals such as an audio signal wave produced in the output or plate circuit of the tube are transmitted to a further amplifier or utilization circuit connected to terminals c-d by way of a low frequency transformer 3| having its primary bypassed for high frequency currents.

If the frequency variations to be detected are of an aperiodic or progressive character, the transformer 3! may be replaced by a resistance coupling network for transmitting the output or control potential to a further network such as the oscillator circuit in a radio receiver for correcting the tuning in a manner known in automatic tuning control (AFC) arrangements. The arrangement shown in Figure 1 may be embodied in the radio frequency section of a tuned radio frequency receiver or amplifier in which case the resonant circuits are variably tuned to the desired central operating frequency, i. e. the unmodulated frequency of the wave to be detected.

upon the grid- If the circuit is embodied in a system for varying operating frequencies, the tuningcondensers II, II, 20 and 22 are preferably ganged to, be operated by a common adjusting member. ,As pointed out 1 above, the tuned circuit l920 which in place of. a parallel tuned or rejector circuit as shown, may be aseries tuned or acceptor circuit or a band-pass type of circuit, is substantially decoupled from the remaining parts of the system and excited substantially by electron coupling through the tube l2 in accordance with the impressed signal variations. It is advisable accordingly to carefully screen this circuit by the provision of a suitable screening device 36 from the remaining circuits especially the tuned transformers H] and 2| provided with further screens 35 and. 31, respectively, .toprevent direct coupling and interaction between these circuits and to ensure stable and eflicient operation of the system. .The screen grid I5 in the example shown has the purpose to prevent the generation of self-lexcited oscillations in the tuned circuit l9.20 by suppressing the effect of inter-electrode capacitative feed-back between the plate I! and the grid I6 to which the tuned circuit Iii-20 is connected. The damping of the tuned circuit |920 is preferably chosen in such a manner as to obtain a maximum conversion efiiciency between the original frequency modulated signals in the input circuit and the.

amplitude modulation in the output circuit, the conversion efficiency being dependent on the damping coefficient of the tuned circuit.

Referring to Figure 2, there is shown a modi fied system embodying a single vacuum tube 12 for converting the frequency modulation to amplitude modulation and for detecting the amplitude modulated signals such as a tube of the diode-pentode type wherein the diode section serves as a detector or demodulator for the amplitude modulated signals. The circuit accordingly differs from Figure lby the provisionof an additional diode anode 40 arranged in co-v operative relation with the common cathode l3 and connected to the plate ll through the, transformer 2! of substantially flat response over the frequency variation range on the one hand and to the cathode through atapped load resistance 4! shunted by a smoothing condenserdZ. In this manner amplitude modulated waves produced in the plate circuit of the tube 12 by the converting action in a manner similar to that according to Figure 1 are impressedupon the diode circuit and demodulated signals, supplied by the diode resistor 4| to the output terminals o-d shunted by additional smoothing condenser 42.

According to a further modification of the invention, the signal waves impressed upon the input grid 14 may be of constant or invariant frequency and the natural or resonant frequency.

of the circuit l9'2ll may be varied such-as by varying either the value of the inductance 19 or of the condenser 29 .(condenser microphone or the like) in which case the inductance or capacity variations are converted into modulations ofan auxiliary signal wave in the output circuit whereby the magnitude of thepotential across the output terminal c.d varies-in proportion to the inductance or capacity variations, respectively. To illustrate this modification of the invention, the condenser 20 is shown in- Figure 1 as being variable, it is understood however that a fixed condenser must be provided if a variable frequency signal is to be detected in the manner .described'hereinabove. In-this manner, the inventive circuit may serve for converting the vibrations of a condenser microphone. into corresponding vibrations of a direct current or to translate slight mechanical movements affecting the capacity of a condenser or the inductance of The neutralizing condenser 2| is adjusted in a known manner so that the bridge comprising the inter-electrode capacity, the neutralizing condenser and the two sections of the coil I9 is balanced and the voltage fed back by the neutralizing condenserv is of the correct amplitude so that the voltage induced by interelectrode capacity is cancelled completely.

Referring to Figure 3, wherein is shown a complete frequency modulation type of receiver designed particularly to receive wide band frequency modulation embodying the principles disclosed in the invention, the signals are received by an antenna and impressed upon a high frequency converter or electronic mixer stage 5! through the transformer which is of design well known and furnishes fiat frequency response over the desired variable frequency range. These signals are beat in a known manner with the signals impressed upon a further control grid of converter valve 5| from the high frequency oscillator 52. The resulting signal is coupled in a manner well known by way ofthe band pass transformer 53 to an I. F. amplifier 54 and from hence impressed upon a limiter 56 through the wide band coupling transformer 55. The limiter is of design well known and eliminates all spurious amplitude modulation from the incoming frequency modulated signal which is to be demodulated. Further control voltage is secured from this limiter to act as a modified type of automatic volume control on the R. F. and I. F. circuits. Voltage from the limiter is fed to the second converter which embodies the principles disclosed by the invention and .de-

scribed in detail hereinabove.

In the same manner as described by the above specification, the frequency modulation is con verted to amplitude modulation in thevalve 5'! and impressed upon the detector .58 which is of the infinite impedance type through the wide band coupling transformer 59. The. output of the detector 58 is suitably filtered and the audio frequency or other signal Voltage free of radio frequency variations is fed to the first audio amplifying valve 60. The output of valve 60 is applied in a well known manner to a power output stage 6i comprising in the example shown a pair of amplifying tubes which are arranged in push-pull and drive the loud speaker 6'2 or the like. There is further shown a feedback circuit from the output of stage (ii to the cathode of the first audio amplifier 63 in order to furnish distortionless audio frequency signals to the loud speaker in a manner well known.

It will be evident from the foregoing that the invention is not limited to the specific details and circuit arrangements disclosed and described herein for illustration, but that the underlying principle of the invention is susceptible of numerous variations and modifications coming within its broader scope and spiritas defined in the ensuing claims. The specification and drawings are accordingly to be regarded in an illustrative rather than a limiting sense.

I claim:

1. A frequency variation response circuit comprising an input circuit carrying high frequency currents, resonant impedance means, the relative frequency of said high frequency currents with respect to the resonant frequency of said impedance means being varied, an electron discharge device comprising a cathode and anode for producing an electron space current and a first and a second control grid, said grids being located at different points in the path of said space current, means for impressing high frequency potential from said input circuit upon said first grid to vary the intensity of said space current in the rhythm of said high frequency currents, means for producing a concentrated electron space charge adjacent to said second grid, further means for connecting said resonant impedance means to said second grid whereby said second grid is excited by said high frequency potential 'by electron coupling with said space charge at a phase varying in sense and magnitude with respect to a normal phase in dependence upon the relative departure of the frequency of said high frequency currents from the tuning frequency of said resonant impedance means, an output circuit for said tube, load impedance means inserted in said output circuit adapted to develop high frequency output energy having an amplitude modulated according to the relative phase departure between the high frequency potentials on said first and second grid, and amplitude modulation detecting means for demodulating said output energy.

2. A frequency variation response circuit comprising an input circuit carrying high frequency currents, resonant impedance means, the relative frequency of said high frequency currents with respect to the tuning frequency of said resonant impedance means being varied, an electron discharge device having a cathode and an anode, a, first control grid located near said cathode and a second control grid located near said anode, means for impressing high frequency potential from said input circuit upon said first grid, means for producing a concentrated electron space charge adjacent to said second grid, further means for connecting said resonant impedance means to said second grid whereby said second grid is excited by said high frequency potential by electron coupling with said space charge at a phase varying in sense and magnitude with respect to a normal phase in proportion to the relative departure of the frequency of said high frequency currents from the tuning frequency of said resonant impedance means, an output circuit connected to said anode, load impedance means inserted in said output circuit adapted to develop high frequency output energy having an amplitude modulated in accordance with the relative phase departure between the high frequency potentials on said first and second grids, and amplitude modulation detecting means for demodulating said output energy.

3. A frequency variation response circuit comprising an input circuit carrying high frequency currents, resonant impedance means, the relative frequency of said high frequency currents with respect to the tuning frequency of said resonant impedance means being varied, an electron discharge device comprising a cathode, an anode, a first control grid and a second control grid, said grids being located at different points in the discharge path between said cathode and anode, means for impressing high frequency potential from said input circuit upon said first grid, a screen grid located between said first and second control grids, means for maintaining said screen grid at a steady positive potential with respect to said cathode to produce a concentrated electron space charge adjacent to said second grid, means for connecting said resonant impedance means to said second grid, whereby said second grid is excited by said high frequency potential by electron coupling with said space charge at a phase varying in sense and magnitude with respect to a normal phase in proportion to the relative departure of the frequency of said signal currents from the tuning frequency of said resonant impedance means, an output circuit connected to said anode, load impedance means inserted in said output circuit adapted to develop high frequency output energy having an amplitude modulated in accordance with the relative phase departure between the high frequency potentials on said first and second control grid, and amplitude modulation detecting means for demodulating said output energy.

4. A frequency variation response circuit comprising an electron discharge tube having means for producing an electron space current and a first and a second lcontrol grid disposed at different points in the path of said space current, means for impressing a high frequency potential of varying frequency upon said first grid, means including resonant impedance means tuned to a predetermined frequency connected to said second grid, whereby said second grid is excited by said high frequency potential by electron coupling with said space current at varying phase in dependence upon the departure of the frequency of said high frequency potential from the tuning frequency of said resonant impedance means, an output circuit for said tube, load impedance means inserted in said output circuit to develop high frequency output energy having an amplitude modulated according to the relative phase departure between the high frequency potentials on said first and second control grid, and amplitude modulation detecting means for demodulating said output energy.

5. A frequency variation response lcircuit comprising an electron discharge tube having means for producing an electron space current and a first and a second control grid disposed at different points in the path of said space current, means for impressing a high frequency potential of substantially constant frequency upon said first grid, a resonant circuit, means for varying the tuning frequency of said resonant circuit, further means including circuit connections from said resonant circuit to said second grid, whereby said second grid is excited by said high frequency potential substantially by electron coupling with said space current at varying phase in dependence upon the relative departure of the tuning frequency of said resonant circuit from the frequency of said impressed potential, an output circuit for said tube, load impedance means inserted in said output circuit adapted to develop high frequency output energy having an amplitude modulated according to the relative phase departure between the potentials on said first and second [control grid, and amplitude modulation detecting means for demodulating said output energy.

6. A frequency variation response circuit comprising an electron discharge tube having a cathode and an anode for producing an electron space current, a. first control grid near said (cathode and a second control grid near said anode, means for impressing a high frequency potential of varying frequency upon said first grid, resonant impedance means tuned to a predetermined frequency connected to said second grid, a screen grid located between said first and second grids, means for maintaining said screen grid at a positive steady potential with respect to said cathode whereby said second grid is excited by said high frequency potential by electron coupling with said space current at varying phase in proportion to the relative frequency departure of the impressed high frequency potential from the tuning frequency of said resonant impedance means, an output circuit connected to said anode, load impedance inserted in said output circuit adapted to develop high frequency output energy having an amplitude modulated in accordance with the relative phase departure between the high frequency potential on said first and second grids, and amplitude modulation detecting means for demodulating said output energy.

7. A frequency variation response circuit comprising an input circuit carrying high frequency currents, resonant impedance means, the relative frequency of said high frequency currents with respect to the tuning frequency of said resonant impedance means being varied, an electron discharge device comprising a cathode, an anode and a first and a'second control grid located at different points in the discharge path between said cathode and anode, means for impressing high frequency potential from said input circuit upon said first grid, further means for producing a concentrated electron space charge adjacent to said second grid, the relation between the frequency of said high frequency currents and the tuning frequency of said resonant impedance means being such that said second grid is excited by said high frequency potential by electron :coupling with said space charge at varying phase in proportion to the relative frequency departure of said high frequency currents from the tuning frequency of said resonant impedance means, an output circuit connected to said anode, load impedance means inserted in said output circuit and adapted to develop high frequency output voltage having an amplitude modulated in accordance with the relative phase departure between the high frequency potentials on said first and second control grids, at least one further auxiliary anode arranged to form a diode discharge path with said cathode, means for impressing said output voltage upon said auxiliary anode, and a diode load circuit connected to said auxiliary anode.

8. A frequency variation response circuit comprising an input :circuit carrying high frequency currents, resonant impedance means, the relative frequency of said high frequency currents with respect to the tuning frequency of said resonant impedance means being varied, an electron discharge device comprising a cathode and an anode for producing an electron space current, a first control grid located near said cathode and a second control grid located near said anode, means for impressing high frequency signal potential from said input circuit upon said first grid, means for connecting said resonant impedance means totsaid second grid, further means for producing a concentrated electron space charge adjacent to said second grid, the relation of the frequency of said signal currents with respect to the tuning frequency of said resonant impedance means being such that said second grid is excited by said high frequency potential by electron coupling with said space charge at varying phase in proportion to the relative frequency departure of said high frequency currents from the tuning frequency of said resonant impedance means, an output circuit connected to said anode, load impedance means inserted in said output cincuit adapted to develop a high frequency output voltage having an amplitude modulated in accordance with the relative phase departure between the high frequency potentials on said first and second grids, amplitude modulation detecting means for demodulating said output voltage, and means for preventing the generation of self-excited 0scillations by said resonant impedance means.

9. In a frequency variation response circuit as claimed in claim 8, said last means comprising a neutralizing circuit between the output circuit and said resonant impedance means to compensate the feed-back potential on said second grid due to inter-electrode capacity therebetween and said anode.

10. In a frequency variation response circuit as claimed in claim 8, said last means comprising a screening electrode between the second grid and said anode.

11. A frequency variation response circuit comprising an electron discharge tube having means for producing an electron space current, a source of high frequency potential, means including circuit connections from said source to said electron tube for causing fluctuations of said space current in accordance with said high frequency potential, a control grid in said tube, means including resonant impedance means connected to said control grid whereby said control grid is excited by said high frequency potential by electron coupling with said space current at varying phase in dependence upon the relative frequency departure of said source from the tuning frequency of said resonant impedance means, an output circuit for said tube, load impedance means inserted in said output circuit adapted to develop high frequency output energy having an amplitude modulated in accordance with the relative phase departure of the potential on said control grid from the fluctuations of said space current, and amplitude modulation detecting means for demodulating said output energy.

. KARL RATH. 

